1. Field of the Invention
This invention relates to methods of and apparatus for concealing errors in digital television signals.
2. Description of the Prior Art
In a digital video tape recorder (DVTR), an incoming television signal to be recorded is sampled, the samples are pulse code modulation coded into digital form, the resulting digital data signals are recorded on a magnetic tape and subsequently reproduced, the reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal. If errors occur in the handling of the digital signals, for example due to noise or tape drop-out occurring in the DVTR, the reproduced digital signals are corrupted and then the re-formed analog television signal does not correspond exactly to the original analog television signal, and a resulting television picture is degraded.
Errors are particularly liable to occur when the DVTR is operating in a special reproduction mode, that is to say in one of the modes which include still, slow motion, reverse motion and the so-called shuttle mode which comprises the whole range of speeds from fast forward to fast reverse.
Such errors and their detrimental effects can be reduced by correction and concealment. Correction involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, it cannot generally be used as the sole means of dealing with errors, because a comprehensive error correction capability would require an excessive amount of additional data which might overload the data handling paths or raise data rate to an unacceptable level. Concealment comprises the replacement of corrupted data signals by available uncorrupted data signals and by data signals generated using available uncorrupted data signals. Concealment relies largely for accuracy on the strong correlation that exists in a television signal.
In concealment, the data signal which is used for replacement of a corrupted data signal can be derived spatially or temporally. Spatial concealment involves the generation of a sample value to replace a sample value which the earlier-applied error correction process has failed to correct, by using available sample values in the same field or frame as the error sample value. To give a very simple example, a concealment sample value can be generated by averaging the two adjacent sample values in the same horizontal scan line. Commonly, however, more sophisticated methods of generating concealment sample values are used, and one such method is disclosed in our UK Pat. No. 2 084 432. Temporal replacement involves the replacement of an error sample value by the sample value from the corresponding sample position in the preceding frame.
Both spatial concealment and temporal replacement have their advantages and disadvantages. Because usually there is little movement in a television picture from frame to frame, temporal replacement more usually gives a better result. However, when movement occurs in the picture, temporal replacement results in a form of picture degradation known as trailing dots, which occur at sample positions in the picture where temporal replacement has been used and hence the picture information has not been updated. Where movement occurs in the picture, and most particularly where there is a lot of movement, such as occurs for example when a television camera is being panned, spatial concealment gives a better dynamic response and hence a picture which is much more acceptable to the human eye. With spatial concealment, however, the overall quality of the picture is reduced.